1. Field of the Invention
The present invention generally relates to an electro-optical reader for reading indicia such as a bar code symbol and, more particularly, to a compact, scan pattern generator for generating an omni-directional scan pattern comprised of multiple scan lines, at least one of which is intended to sweep over a symbol associated with a target or product presented to the reader.
2. Description of the Related Art
Flat bed laser scanners, also known as slot scanners, have been used to electro-optically read bar code symbols, particularly of the Universal Produce Code (UPC) type, at a point-of-sale in supermarkets, warehouse clubs, department stores and other kinds of retailers for many years. One kind of slot scanner has a single, horizontal window set flush with, and built into, a countertop of a checkout station. Products to be purchased bear an identifying symbol and are slid across the horizontal window through which a multitude of scan lines is projected in a generally upwards direction. The scan lines are generated by a scan pattern generator which is designed to scan a symbol on a bottom side and/or a leading vertical side of each product presented to the horizontal window.
Another type of slot scanner has two built-in windows, one is horizontal as described above, and the other is vertical and positioned adjacent the horizontal window. Additional scan lines are projected through the vertical window to scan additional vertical sides of each product presented, thereby minimizing the need for a checkout clerk to visually locate the symbol on each product and rotate the product to a specific location where the symbol is best positioned to be read.
Still other types of scanners capable of generating multiple scan lines are portable units, either hand-held during reading, or placed on a countertop to serve as a workstation. The portable units have windows which are aimable at the symbol to be read and are more versatile than built-in installations.
As exemplified by U.S. Pat. No. 5,059,779; U.S. Pat. No. 5,124,539 and U.S. Pat. No. 5,200,599, the known scan pattern generators generate the scan lines in complex patterns of intersecting lines for superposition over the symbol to be read to insure that, no matter what the angular orientation of a respective symbol might be within predetermined limits, at least one of the scan lines or part of the pattern will be scanned over the entire length of the respective symbol.
FIG. 1 depicts a known scan pattern generator 10 in accordance with the prior art, in which a laser 12 emits a laser beam at a mirrored component 14 mounted on a shaft 16 for rotation by a motor 18 about an axis 20 in direction of the arrow 22. The component 14 has a plurality of flat main mirrors 30, 32, 34, 36 (as shown, four) and is known as a mirrored polygon. Each main mirror is tilted relative to the axis 20. A plurality of side mirrors 40, 42, 44, 46, 48 (as shown, five) is arranged about the axis 20.
During rotation of the polygon, a portion of the main mirror 30 adjacent its leading edge 50 reflects an incident light beam from the laser 12 onto side mirror 40 for reflection therefrom to the symbol. As main mirror 30 turns, the incident beam from the laser is successively reflected onto side mirrors 42, 44, 46, 48 until the trailing edge 52 clears the laser beam. The main mirror 30 thus generates a set of five scan lines. Similarly, each successive main mirror 32, 34, 36 generates a respective set of five scan lines. The overall scan pattern consists of four sets of five scan lines each. The length of each scan line depends on the length of each mirror. The angle of inclination of each mirror determines the extent to which the scan lines intersect one another and their relative orientation.
Although generally satisfactory for their intended purpose, the known scan pattern generators have a small number of scan line orientations, thereby limiting their performance especially on height-truncated symbols. To increase the number of scan line orientations, additional space is required to accommodate additional side mirrors. However, additional space is not readily available in hand-held portable units. The number of side mirrors could be increased by making each side mirror shorter in length; however, this would decrease scan line length and compromise performance.